Black carbon larger cause of climate change than previously assessed

A new international study found that soot, or “black carbon,” is second only to the greenhouse gas carbon dioxide as a climate warming agent.
Credit: NASA

Soot, known as black carbon to scientists, is the second largest man-made contributor to global warming, and its influence on climate has been greatly underestimated, according to a new international study published in the Journal of Geophysical Research-Atmospherestoday.

Diesel engines, forest fires and many other sources throw heat-trapping specks of soot – black carbon – into the atmosphere. In addition to causing respiratory health problems, black carbon also warms the climate. For decades, its full impact on climate has been the source of much debate.

The study for the first time presents a comprehensive and quantitative analysis of the role of black carbon on the climate system. David Fahey, Ph.D., a research physicist in the NOAA Earth System Research Laboratory in Boulder, Colo., is a co-lead author of the study, led by the International Global Atmospheric Chemistry Project (IGAC).

The research indicates that black carbon ranks second behind carbon dioxide as the major cause of man-made global warming and that its influence on climate has been underestimated, confirming some earlier studies that also showed a significant role for black carbon in climate warming. The study, a four-year, 250-page effort, is likely to guide research efforts, climate modeling, and policy for years to come.

"This study confirms and goes beyond other research that suggested black carbon has a strong warming effect on climate, just ahead of methane," says Fahey. In fact, the best estimate of direct climate influence by black carbon in this report is about a factor of two higher than most previous work, including the estimates in the last Intergovernmental Panel on Climate Change (IPCC) Assessment released in 2007, which were based on the best available evidence and analysis at that time. Scientists have spent the years since the last IPCC assessment improving estimates, but the new IGAC assessment notes that emissions in some regions are probably higher than estimated. This is consistent with other research that also hinted at significant under-estimating for some regions' black carbon emissions.

The results indicate that there may be a greater potential to curb warming by reducing black carbon emissions than previously thought.

Exhaust from a diesel truck.

Credit: EPA

The international team urges caution because the role of black carbon in climate change is complex. "Black carbon influences climate in many ways, both directly and indirectly, and all of these effects must be considered jointly," says co-lead author and snow measurement expert Sarah Doherty, Ph.D., of the NOAA Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington. The dark particles absorb incoming and scattered heat from the sun); they can promote the formation of clouds that can have either a cooling or warming impact; and black carbon can fall on the surface of snow and ice, promoting warming and increasing melting. In addition, many sources of black carbon also emit other particles, which counteract black carbon providing a cooling effect.

The research team quantified all the complexities of black carbon and the impacts of co-emitted pollutants for different sources, taking into account uncertainties in measurements and calculations. The study suggests mitigation of black carbon emissions for climate benefits must consider all emissions from each source and their complex influences on climate.

In addition, the report finds black carbon is a significant cause of the rapid warming in the Northern Hemisphere at mid to high latitudes, including the northern United States, Canada, northern Europe and northern Asia. Its impacts can also be felt farther south, inducing changes in rainfall patterns from the Asian Monsoon.

The new assessment, "Bounding the role of black carbon in the climate system: A scientific assessment," is published online at the Journal of Geophysical Research-Atmospheres, a journal of the American Geophysical Union, and can be accessed free of charge.

The International Global Atmospheric Chemistry (IGAC) Project of the International Geosphere-Biosphere Programme coordinates and fosters atmospheric chemistry research worldwide. IGAC is a community of over 3,000 academic and government scientists, students, policy makers and others from six continents.

NOAA's mission is to understand and predict changes in the Earth's environment, from the depths of the ocean to the surface of the sun, and to conserve and manage our coastal and marine resources. Visit us at www.noaa.gov and join us on Facebook, Twitter and our other social media channels.